There really can't be any guarantees unless the developer states they'll support Windows 8. There's always been "little" things that creep up in the past, like some crashes that would happen on Windows 7 but not Windows Vista / XP for a game developed before Windows 7. As botat29 mentioned, drivers also play a large role, and you'll obviously have less driver versions to work with on 8 than on 7 for now. (e.g. You can't revert to older drivers if something in the newest driver breaks your older game.)

Had a snag installing Steam to an entry level Lenovo W8 laptop, it kept hanging up during the final update process before you log in for the first time. Running as administrator on the 3rd try got it going fine, (or it finally was able to connect and update). Haven't tried installing any games, afaik if a game doesn't say Win8, don't count on it working, plain and simple.